Convert int to ASCII and back in Python

Question

I'm working on making a URL shortener for my site, and my current plan (I'm open to suggestions) is to use a node ID to generate the shortened URL. So, in theory, node 26 might be short.com/z, node 1 might be short.com/a, node 52 might be short.com/Z, and node 104 might be short.com/ZZ. When a user goes to that URL, I need to reverse the process (obviously).

I can think of some kludgy ways to go about this, but I'm guessing there are better ones. Any suggestions?

Answer 1

ASCII to int:

ord('a')

gives 97

And back to a string:

• in Python2: str(unichr(97))
• in Python3: chr(97)

gives 'a'

Answer 2

>>> ord("a")
97
>>> chr(97)
'a'

Answer 3

If multiple characters are bound inside a single integer/long, as was my issue:

s = '0123456789'
nchars = len(s)
# string to int or long. Type depends on nchars
x = sum(ord(s[byte])<<8*(nchars-byte-1) for byte in range(nchars))
# int or long to string
''.join(chr((x>>8*(nchars-byte-1))&0xFF) for byte in range(nchars))

Yields '0123456789' and x = 227581098929683594426425L

Answer 4

What about BASE58 encoding the URL? Like for example flickr does.

# note the missing lowercase L and the zero etc.
BASE58 = '123456789abcdefghijkmnopqrstuvwxyzABCDEFGHJKLMNPQRSTUVWXYZ' 
url = ''
while node_id >= 58:
    div, mod = divmod(node_id, 58)
    url = BASE58[mod] + url
    node_id = int(div)

return 'http://short.com/%s' % BASE58[node_id] + url

Turning that back into a number isn't a big deal either.

Answer 5

Use hex(id)[2:] and int(urlpart, 16). There are other options. base32 encoding your id could work as well, but I don't know that there's any library that does base32 encoding built into Python.

Apparently a base32 encoder was introduced in Python 2.4 with the base64 module. You might try using b32encode and b32decode. You should give True for both the casefold and map01 options to b32decode in case people write down your shortened URLs.

Actually, I take that back. I still think base32 encoding is a good idea, but that module is not useful for the case of URL shortening. You could look at the implementation in the module and make your own for this specific case. :-)

Answer 6

apparently I'm late to the party, jus like to share a snippet I use very often.

/**
 * 62 = 26 + 26 +10
 *
 * @param id
 * @return
 */
public String base62(long id) {
    StringBuilder sb = new StringBuilder();
    while (id >= 62) {
        int remainer = (int) (id % 62);
        id = id / 62;
        sb.append(index2char(remainer));
    }
    sb.append(index2char(id));

    return sb.reverse().toString();
}

public long reverseBase62(String s) {
    long r = 0;
    for (int i = 0; i < s.length(); i++) {
        r = r * 62;
        int index = char2index(s.charAt(i));
        if (index == -1) {
            throw new IllegalArgumentException(
                String.format("[%s] is in malformation, should only contain 0~9, a~z, A~Z", s));
        }
        r += index;
    }

    return r;
}
private char index2char(long index) {
    if (index < 10) {
        return (char) ('0' + index);
    }
    if (index < 36) {
        return (char) ('a' + index - 10);
    }
    return (char) ('A' + index - 36);
}



private int char2index(char c) {
    if ('0' <= c && c <= '9') {
        return c - '0';
    }
    if ('a' <= c && c <= 'z') {
        return c - 'a' + 10;
    }
    if ('A' <= c && c <= 'Z') {
        return c - 'A' + 36;
    }
    return -1;
}